Which combination is used to convert a carboxylic acid to an ester?

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Esterification of a carboxylic acid relies on an alcohol supplying the alkoxy group that ends up in the ester. Under acidic conditions, the carboxyl group reacts with the alcohol to form an ester and water (Fischer esterification). Mechanistically, the carbonyl is protonated, the alcohol attacks the carbonyl carbon, a tetrahedral intermediate forms, water is expelled after proton transfer, and the product is deprotonated to give the ester. Because this reaction is driven by removing water and/or using excess alcohol with an acid catalyst, alcohol is the reagent that actually enables the transformation. The other options don’t enable this conversion: an ester is already the product and can’t convert a carboxylic acid to another ester under the same conditions; a carboxylic acid is the starting material, not the reagent; an amide would lead to amide formation rather than the ester linkage.